This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Oral capecitabine will be well tolerated by pediatric patients with newly diagnosed nondisseminated, intrinsic brainstem gliomas and non-disseminated high-grade gliomas. ISPECIFIC AIMS Primary Objectives: 1.To estimate the maximum tolerated dose (MTD) of capecitabine administered concurrently with radiation therapy (RT) to children with newly diagnosed nondisseminated, intrinsic brainstem gliomas or newly diagnosed non-disseminated high-grade gliomas. 2.To describe the dose-limiting toxicity (ies) of capecitabine administered concurrently with radiation therapy to children with newly diagnosed nondisseminated, intrinsic brainstem gliomas or newly diagnosed non-disseminated high-grade gliomas. Secondary Objectives: 1.To characterize the pharmacokinetics of capecitabine as delivered by Capecitabine Rapidly Disintegrating Tablets in this pediatric patient population. 2.To describe in the context of this phase 1 investigation, the anti-tumor activity of capecitabine and radiation that is observed in children with newly diagnosed nondisseminated, intrinsic brainstem gliomas or newly diagnosed non-disseminated high-grade gliomas. 3.To characterize radiographic changes in non-disseminated, newly diagnosed intrinsic brainstem gliomas and high-grade gliomas treated with radiation and capecitabine using MRI, MRS, perfusion and diffusion imaging and PET scans.